[wallet] Schedule tx rebroadcasts in wallet

Removes the now-unused Broadcast/ResendWalletTransactions interface from
validationinterface.

The wallet_resendwallettransactions.py needs a sleep added at the start
to make sure that the rebroadcast scheduler is warmed up before the next
block is mined.

test/functional/wallet_resendwallettransactions.py

@@ -39,6 +39,12 @@ class ResendWalletTransactionsTest(BitcoinTestFramework):
         self.log.info("Create a new transaction and wait until it's broadcast")
         txid = int(node.sendtoaddress(node.getnewaddress(), 1), 16)
 
+        # Wallet rebroadcast is first scheduled 1 sec after startup (see
+        # nNextResend in ResendWalletTransactions()). Sleep for just over a
+        # second to be certain that it has been called before the first
+        # setmocktime call below.
+        time.sleep(1.1)
+
         # Can take a few seconds due to transaction trickling
         wait_until(lambda: node.p2p.tx_invs_received[txid] >= 1, lock=mininode_lock)